Optimize system efficiency, size, and cost using fixed-output bus converters

Most telecom and industrial systems use a power architecture like that shown in Figure 1,

Figure 1: To keep power-system design simple, ac power is converted to a 48 V, dc-distribution voltage. Each system board converts the 48 V to an intermediate dc-bus voltage, for example 12 V, which is fed to non-isolated point-of-load buck converters.
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where ac power is converted to a system-wide 48 Vdc distribution bus, which is then converted at each application board to a 12 V intermediate bus, which is again converted down to the core application voltages (1.8 V, 2.5 V, 3.3 V, etc.) The advantage of this architecture is that each link in the modular power chain is in itself very simple, reducing application design time, and time to market.

The first link in the chain is the standard, safety-agency approved ac/dc converter (often called a silver box), which generates a 24 Vdc or 48 Vdc distribution bus. The 48 V bus voltage is high enough to be a relatively efficient distribution voltage, but it is also safe, allowing engineers who don't have expertise in high-voltage safety equipment or procedures to design board-level power converters.

To further simplify board design, the 48 V input is converted to a non-isolated, 12 V intermediate dc-bus voltage on an application board. The 12 V value is not required, nor is it always the best choice for an intermediate bus; it is just the most commonly used voltage. The 12 V bus voltage is then converted to core voltages such as 3.3 V, 2.5 V and 1.8 V with simple, non-isolated buck converters.

An intermediate bus converter can produce a constant output voltage or it can ratiometrically track the input (direct ratio converter). A constant-output converter must monitor and regulate the output voltage, which can result in higher converter cost relative to a direct-ratio converter. A direct-ratio converter provides no regulation; it simply generates an output voltage that is proportional to the input voltage. The direct-ratio converter produces an output voltage range with the same ratio (usually 2:1) as the input voltage range.

For example, if the input voltage varies over 36 to 72 V range, the converter output may vary over 8 to 16 V range. If a direct-ratio bus converter is used, the buck converters, which produce 1.8 V, 2.5 V, 3.3 V, etc. must be able to take the entire bus converter output range as input. Both regulated and direct-ratio converters simplify the system design, but close attention should be paid to overall system performance when choosing the type of converter.

System Efficiency Considerations
Modular simplicity and expandability are hallmarks of the power distribution architecture shown in Figure 1, but such ease-of-design comes at a cost. The main downside is that three power converters connected in series diminishes the overall system efficiency as shown by:

ηSYSTEM = ηac - dc· ηBUS· ηBUCK
where η = efficiency

For example, if the efficiency of each block in Figure 1 is 90%, the overall system efficiency is 73%. If, however, the efficiency of each converter stage is improved by 1%, the overall efficiency jumps by 2.3% to 75.3%. It's not hard to see that in a large distributed system, with dozens or hundreds of boards, a small improvement in efficiency at each conversion stage adds up to a significant power savings.

Overall system performance depends on the interaction of the bus converter and buck converters. If the bus converter produces a fixed output, the buck converter efficiency, size, and cost can be optimized for the known bus voltage. Optimization may not be as easy with a direct-ratio bus converter because the buck converters must operate over wider input voltage range.

Figure 2 shows a typical buck converter,

Figure 2: A typical synchronous buck converter is very simple and can generate low-voltage rails with efficiencies over 90%.
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This circuit is the model for the performance analysis presented here. The converter in Figure 2 operates over 5 to 24 V input range and produces 2.5 V/10 A output. The same circuit can be used to generate other core voltages, in the range from 0.8 V to 0.9 · VIN. To keep the analysis relatively simple, we will use four identical buck converters (Figure 2) all producing 2.5 V/10 A output, for a total of 100 W of power. The analysis can be easily adjusted for any other combination of output voltages and loads. Also, to limit the scope of this article, we leave out the performance of ac/dc converter, which can be factored in at any time using the principles presented here.

Choosing a Bus Voltage
What bus voltage is best? For most design engineers the answer is simple: use 12 V, because that is what was used on the last project. Unfortunately, that choice may result in lower system efficiency, hotter circuit operation and lower reliability. Choosing the optimally efficient bus voltage requires a little more work, including measuring the efficiency of the bus converter at various outputs and measuring the efficiency of the buck converters at various inputs.

Figure 3 shows an example of the latter data point. Buck IC manufacturers usually do not supply this information so it may be necessary to do the measurements in-house. Efficiency should be plotted for the real-world, worst-case load current, which may not be the maximum allowable output current of the buck converter.

The second step is to compile a set of bus converter efficiency curves for different output voltages, Figure 4.

Figure 5: This simple 12 V bus converter has peak efficiency of 95%, operates over full 36 to 72 V telecom input, and is easy to modify for different output voltages.
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The converters for the remaining output voltages are essentially the same circuit as that shown in Figure 5 except each is optimized with different transformers, output MOSFETs, and inductors.

The third step is to calculate the system efficiency for each combination of bus converter and buck converter. For four buck converters in Figure 2, each producing 2.5 V/10 A (25W of output power), the total load presented to each bus converter is:

PBUS = POUT/ηBUCK
where PBUS is the output power required from bus converter, and POUT is the cumulative output power of all dc/dc buck converters. In this example, POUT is 4 · 25 W or 100 W.

The last step is to calculate the required output current of each bus-converter candidate:
IBUS = PBUS/VBUS

For instance, a 5 V bus converter must supply 21.2 A to provide enough power to the load, considering the efficiency of the four buck units. Once this bus-converter output current is calculated, we can look up the bus-converter efficiency at given output load and calculate total system efficiency:

ηSystem = ηBUS· ηBUCK

As discussed earlier, to make the analysis simpler, the ac/dc converter efficiency is left out of these calculations; this efficiency is usually fixed by the choice of off-the-shelf silver box. The calculated efficiency versus bus voltage is plotted in Figure 6.

Figure 6: The overall system efficiency is highest for bus voltage of 7 V. Different converters may result in a different optimal bus voltage, and should be carefully evaluated to obtain highest system efficiency.
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It may come as a surprise that the highest system efficiency occurs around 7 V, since that is almost never used as a bus voltage. By using the above analysis, it is easy to find out what configuration of converters will perform best at system level.

Conclusion
By carefully considering the whole power system, efficiency can be maximized by selecting the optimal bus voltage for a particular application, by using a few measurements and some simple calculations. In the end, an optimized bus converter may be smaller, more flexible, require less cooling, and be less expensive than an off-the-shelf 12 V power module, thus meeting even most demanding application specifications.

About the authorGoran Perica is a Senior Applications Engineer at Linear Technology Corp., Milpitas, CA, where he has worked for nine years. In his current position, he designs power-conversion circuits for the telecommunications, computer, and industrial markets. He has a Master's Degree in Electrical Engineering from the University of Ljublana, Slovenia. His spare time pursuits include enjoying music and hiking with his wife and three ex-racing greyhounds.